Microbiology

Bacteria produce oxygen even without light

The molecular secrets of a bacterium which produces its own oxygen to use the green house gas methane have been unravelled

March 25, 2010

Dutch researchers from the University of Nijmegen have discovered bacteria that oxidize the methane without oxygen. Instead, these bacteria used nitrite, commonly available in freshwater sediments in agricultural areas. Methane is a very stable molecule and its degradation was generally believed to be impossible without oxygen (or sulfate). Now an international team from the Netherlands, France and Germany has shown that the bacteria actually do use oxygen for methane oxidation. They produce this oxygen themselves, like plants - only without light. The oxygen is manufactured from the nitrite. Until now, scientists believed that the art of making oxygen was restricted to plants, algae and cyanobacteria. Now it looks as if the researchers are on the track of a mechanism which may have existed before green plants first appeared on earth. (Nature, March 25th 2010)

The unravelling of the new oxygen producing pathway was difficult because the responsible microbe grows only very slowly and remained hidden inside a complex microbial community. For this reason, short DNA fragments were extracted from the community as a whole and sequenced with modern massive parallel sequencing technology. From these fragments the genome of the responsible bacterium could be stitched together. This demanding approach has been successful only a few times before. It was achieved by Denis Le Paslier and colleagues of Genoscope (Evry, France).

The genome showed very clearly that the known genes for N2O reduction were missing and that the organism was genetically dependent on oxygen. "The experimental and genetic data were clearly incompatible" says Marc Strous, who led the research effort in Nijmegen and has moved to the Max Planck Institute in Bremen in the meantime.

Given these circumstances, how was the organism able to obtain its energy from the oxidation of the relatively inert molecule methane (CH4) with nitrite (NO2-) as electron acceptor? That is like starting a fire under water. To solve this paradox, Marcel Kuypers and colleagues of the Max Planck Institute for Marine Microbiology were called to the rescue. Advanced microsensing and mass spectrometry confirmed that the paradox was real - both data were right and could only be explained by a new way of oxygen production. After one year of trying, Katharina Ettwig, who hopes to graduate on this work this year, was able to actually trap the oxygen and provide the experimental proof. She named the organism Methylomirabilis oxyfera (wonderful methane-eater making oxygen), as it uses two nitrogen monoxide molecules to produce nitrogen and oxygen which is then used to attack the inert methane molecule.

The scientists suggest that the newly discovered pathway of oxygen production may be a missing link that once, billions of years ago, made possible the evolution of oxygenic photosynthesis, now performed by plants. But it certainly forces a rethink of current understanding of the role of fertilizers in the methane cycle.